A general problem encountered in the manufacture of elongated incandescent tubular lamps using a coiled filament is that of providing proper centering support for the filament. In tubular lamps of the double-ended type having an axially extending filament, particularly one possessing a relatively large diameter and connected at its opposite ends to lead-in conductors sealed through respective opposite ends of the envelope, it has been the practice to provide either a threaded or a tapered insert with associated ring spacer. Each of these in turn is located at one end of the filament to support the filament from the envelope walls and maintain the filament generally at the axis of the tubular envelope. The insert typically has one end secured to the lamp sealing foil and has either the threaded or tapered portion at the other end. The ring spacer is a separate piece secured to the insert, typically at the mid-point thereof. The threaded or tapered portion is adapted to fit within an end of the helically coiled filament.
The filament support comprised of an insert and associated ring spacer has certain disadvantages associated therewith. Because of the use of two separate parts, there is an extra step involved in assembly and welding these parts together. Furthermore, the welded junction creates oxide clean-up problems. Also, because the insert is disposed within the filament coil end, the depth of insertion into the filament is difficult to control and measure because the insert is essentially out-of-sight within the filament coil. This creates variations in insertion depth which is a major factor in inconsistencies in lamp performance.
A number of prior art techniques exist for providing filament support in tubular lamps. For example, U.S. Pat. Nos. 3,736,455 and 3,820,207 show filament support structures. However, these support structures are not primarily for end filament support and it is noted that in both of these references, the support member requires separate connection to the glass seal. For example, in U.S. Pat. No. 3,736,455, it is noted that at each end of the lamp there are utilized (and required) two support foils.
In U.S. Pat. Nos. 2,813,993, 3,194,999, 3,195,000; and 4,310,782, there are illustrated filament supports that are basically in the form of a support ring, usually disposed at either an intermediate position or at spaced positions along the filament. These support rings do not provide for both filament support and insertion depth control. Moreover, these patents do not show a filament support that is of single-piece integral construction and that additionally forms an inner power lead.